The invention relates to ring interferometers for measuring the effect of nonreciprocal phenomena such as the Faraday effect and the Sagnac effect with very great sensitivity and very great stability.
In a ring interferometer, or Sagnac interferometer, two beams travel in opposite directions over the same optical path and interfere at the exit from this path. As long as a disturbance in this path presents the same characteristics in both porpagation directions and does not vary during the transit time of the light in the interferometer, the two beams are affected identically and their relative phase remains unchanged. Disturbances of this type are called "reciprocal". Because the transit time in an interferometer is generally very small, the variations of a disturbance during this time, except if this latter is introduced voluntarily, are generally negligible.
But there exist "nonreciprocal" disturbances which have a different amplitude in the two propagation directions, these are physical effects which, by establishing its complete orientation, destroy the symmetry of the space or the medium.
Two known effects present this property:
the Faraday effect, or colinear magneto-optical effect, where a magnetic field creates a preferential orientation of the spin of the electrons of the optical material,
the Sagnac effect, or relativistic inertial effect, where the rotation of the interferometer with respect to a Gallilean reference destroys the symmetry of the propagation times.
In a ring interferometer, only "nonreciprocal" disturbances of this type have an effect on the detected signal. The dimensional variations such as creep, thermal expansion, pressure variation, or the refraction index variations have not, as far as they are concerned, any effect on the detected signal. We have then, in principle, an instrument for measuring "nonreciprocal" effects which presents a perfect stability.
In practice, so that the reciprocal disturbances may have a strictly zero effect, the two beams of the interferometer must travel exactly over the same path. More precisely, the two waves must have two identical solutions of the wave equation of the interferometer, the sign of the "time" parameter being reversed.
When the interferometer is constructed for free propagation, and this is the case when using discrete optical elements, this condition is never strictly respected:
the wave equation presents a "continuum" of solutions and the least disalignment of the optical means leads to different solutions, so non-superimposed wave fronts being obtained;
even for identical solutions, when infinite extension waves are considered, plane waves for example, the intensity distribution, necessarily limited in practice, differs, in fact, were it only because of the diffraction, and disrupts the reciprocity.
A monomode type solution consists in an interferometer having an end-to-end wave-guide structure, and described in French patent application No. 77 35039, filed on Nov. 22, 1977. In this case, the wave equation presents a discrete number of solutions and it is possible, in principle, to use the same one of these solutions, or modes, in each of the two propagation directions. However, because couplings between modes are always present, it is preferable for the guide structure to be monomodal. But this solution is technologically difficult to use.